DE-OS No. 2,458,380 discloses dental filling materials which contain oxides and/or carbonates of lanthanum, hafnium, strontium and/or tantalum as radiopaque fillers. This publication also discloses that it is known that elements with a high atomic weight have a stronger X-ray absorption. However, there are limits to the uses thereof in the vicinity of the human tissue, because many of these heavy elements are toxic or radioactive.
DE-OS No. 2,420,531 also discloses dental filling materials, which can contain as radiopaque material barium sulphate, tantalum, iodalphionic acid, iopanionic acid or ipodoic acid. Reference is also made to the other radiopaque materials described in Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd edition, Vol. 17, pp. 130-141.
European Pat. No. 11,735 describes dental filling materials containing as the contrast medium compounds of barium, bismuth, lanthanum, thorium and rare earth metals, preference being given to the use of barium sulphate.
It is known from other publications to incorporate methacrylate particles of aliphatic halogen compounds, such as e.g. tetrabromomethane, bromoform, ethyl iodide, iodobenzene, etc. This is brought about by suspension polymerization of an alkyl methacrylate. There can be an adhesive pigment of a heavy metal compound on the beads for increasing the radiopacity.
The literature also mentions dental materials, which e.g. contain tin compounds, barium glasses, barium sulphate, etc as the radiopaque agent. A description is also provided of glass ceramics containing lanthanum, zirconium, strontium, tantalum or hafnium in the form of their oxides, carbonates or fluorides. It is stated in DE-OS No. 2,935,810, that although oxides of rare earths (elements 57 to 71) have been proposed as impervious to X-rays, but problems then occur due to undesired discolouration.
It has also already been proposed to use calcium, strontium, barium, lanthanum, rare earths, tantalum and/or hafnium aluminosilicate with a zeolite structure as the filler for a dental material.
In addition, radiopaque filling materials are known, whose filler consists of a mixture of microfine silicon dioxide and radiopaque glasses, the latter having an average particle size over 1 .mu.m. All the hitherto known radiopaque microfilled dental materials suffer from the disadvantage that the radiopacity is still unsatisfactory. In addition, the transparency of the known radiopaque microfilled dental materials is unsatisfactory and the high-lustre polishability inadequate.